Have you ever had to deal with college students parents?  Generally, when you deal with college students parents, it is over a grade or a policy.  Dealing with college students parents can be a challenge.  I recommend that you keep ACCURATE documentation of all assignments, projects, tests, quizzes, etc., and you show the students and parents how the grade was calculated based on graded tests, assignments, quizzes, projects, etc.  It is important not to discard graded papers for at least one extra grading period, in the event you need to refer to the graded papers.  Remember documentation is all you have as proof, and when you have accurate documentation, neither the parents nor the students have a leg to stand on.  Accurate documentation has saved many instructors.  When it comes to policy, you need to ensure that you have enforced the policy as it is stated in writing, and the writing will serve as documentation.  When you have enforced the policy as it is documented, neither the parents nor the students have a leg to stand on.  Documentation is definitely a saving grace in the education arena.